Recently, many countries throughout the world have tried to develop wind power, tidal power, geothermal power, solar power, and hydrogen gas, etc. as an energy source for replacing depletive fossil fuel. Among them, hydrogen gas has the highest energy efficiency per unit mass and no harmful byproducts during combustion, and thus, research on the preparation, storage, and transportation, etc. thereof has been conducted. In particular, in accordance with the practical use of a fuel cell, a material capable of efficiently storing the hydrogen gas has been developed.
Currently, materials capable of storing the hydrogen gas include metal hydride, ammonia borane (NH3BH3), carbon nanotube, carbon compounds such as active carbon, zeolite, a metal-organic framework (MOF), a covalent organic framework (COF), and the like. Among them, when it is assumed that ammonia borane uses all 3 equivalent of hydrogen, ammonia borane can store 19.6 wt % of hydrogen, such that a high hydrogen storage amount is exhibited. Accordingly, a method for storing and transporting hydrogen by using ammonia borane has been actively studied. However, the largest problem of practical use of ammonia borane is to regenerate the used ammonia borane. If hydrogen is removed from ammonia borane, a high molecular weight material with very large viscosity such as polyborazylene as well as a volatile material such as borazine is generated, and thus, it is very difficult to regenerate ammonia borane by adding hydrogen again. This regeneration problem is one of the largest reasons for classifying ammonia borane as only an offboard hydrogen storage material, not an onboard hydrogen storage material.
The metal-organic frameworks (MOFs) capable of storing hydrogen by a physical adsorption among other hydrogen storage materials are a kind of organic-inorganic hybrid compound, and are a material in which metal and an organic ligand are three-dimensionally linked to each other, and the organic ligand is used as a linker. Specifically, as shown in FIG. 1, the MOFs means a material in which the organic ligand is coordinated to two or more metals, and each of the coordinated metals is serially coordinated to one or more other organic ligands, thereby forming many tiny spaces, i.e. a network structure with pores, inside the framework.
This metal-organic framework is manufactured by various producing methods. For example, the MOFs can be prepared through a substitution reaction of an organic ligand ion by using metal salt as a metal source. In detail, in such preparation, zinc nitrate [Zn(NO3)2] as a metal source, and a dicarboxylic acid compound as a ligand are mainly used so as to prepare the framework (O. M. Yaghi et al. Science, 2003, vol. 300, p. 1127; WO 02/088148).
In addition, there is a method for preparing an isoreticular metal-organic framework (IRMOF) by using zinc as a metal source to thereby form zinc oxides (Zn4O) as a core and by using an organic ligand such as a dicarboxylic group. Further, there is a method for preparing a metal-organic framework by using, instead of zinc, the metal ion such as Cu and Fe as a core, and using tridentate or multidentate organic ligands.
As another porous crystal structure, many efforts for forming a covalently bonded network structure composed of only organic materials have been made, and recently, a group of professor, Yaghi at the University of California, Berkeley synthesized a covalent organic framework including a cluster of boron and presented the contents thereof [US2006/0154807 A1]. According to the contents of the patent, it is possible to constitute a network formed of a covalent bond by bonding each bonding group with at least two clusters of boron.
In the real synthesis, a two-dimensional plane network is generated by a polycondensation reaction of a benzene diboronic acid (BDBA), lamination is formed through an interaction between the generated plane networks, and thus, the covalent organic framework has crystallinity. In this case, a length of an entrance of the generated pore is about 15 Å.
However, the storage capacity of the physical adsorption hydrogen storage material is largely insufficient in order to be commercialized, and particularly, since the storage capacity of the material at room temperature does not approach 1 wt % even under high pressure, and thus, a groundbreaking idea is required for largely increasing capacity.
In a prior art, there was an effort for changing a hydrogen storage property of ammonia borane by including ammonia borane in a porous material such as zeolite, but since ammonia borane is physically included in the pore of the porous material and the property of the crystal structure is not substantially changed, there is a performance improvement in that a speed of a dehydrogenation reaction is increased, but a regeneration problem of ammonia borane is still not solved.